Pacorus I (died 38 BC) was a Parthian prince, who was the son of king Orodes II and queen Laodice. It is possible that Pacorus was co-ruler with his father for at least part of his father's reign. His wife was an unnamed Armenian princess, who was one of the daughters of King Tigranes the Great of Armenia and his wife, Queen Cleopatra of Pontus.

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Pacorus is first mentioned in 51 BC, then probably about 12 years old[citation needed], as the head of a Parthian army during an invasion of the Roman province of Syria. However, because of the young age of Pacorus, the Parthian operations were probably led by the Parthian commander Osakes. According to the Roman historian Cassius Dio,[1] the Parthian army besieged Antioch, but were unable to capture the city and withdrew. During this event, Osakes was killed during a Roman counter-attack under the Roman general Gaius Cassius Longinus, which made the Parthian troops under Pacorus retreat back to their homeland.

Some time later, when the Roman consulMarcus Calpurnius Bibulus, was appointed the governor of Syria, tried to incite the Parthians against each other; he had close friendship with a Parthian satrap who harbored a grudge against king Orodes II, and made him crown the young Pacorus as rival-king of the Parthian Empire and made a campaign against Orodes. Even coins with the portrait of Pacorus were briefly minted. However, Pacorus and his father reconciled to each other and coins with the portrait of Pacorus were stopped minting. The Parthian satrap who was behind the plot was probably shortly executed.

In 42 BC, Quintus Labienus, a Roman rebel, began serving the Parthians.[2][3] Later in 40 BC, the Parthians under Pacorus and Labienus invaded the Roman territories. The Parthian army crossed the Euphrates and attacked Apamea. The attack on Apamea failed but Labienus was able to entice the Roman garrisons around Syria to rally to his cause. The combined Romano-Parthian army then proceeded to defeat Mark Antony's governor L. Decidius Saxa in a pitched battle and took Apamea. After the Roman defeat at Apamea, the Parthians split their army. Pacorus turned south and conquered the Levant from the Phoenician coast through Palestine. Labienus turned north to follow Saxa, whom he defeated and killed in Cilicia. In Judea, Pacorus' deputy Barzapharnes deposed king Hyrcanus II and appointed the latter's nephew Antigonus as king in his place.

Under Labienus and Pacorus, the Parthians restored their territory to nearly the limits of the old Achaemenid Empire and controlled all of Asia Minor except for a few cities but the Parthian successes were not long-lasting. In 39 BC, a Roman counterattack under Publius Ventidius Bassus in Asia Minor defeated Labienus, who was subsequently captured and executed.

After the disastrous campaign in Asia minor, the Parthians launched another invasion into Syria in 38 BC, led by Pacorus. Ventidius, in order to gain time, leaked disinformation to Pacorus implying that he should cross the Euphrates River at their usual ford. Pacorus did not trust this information and decided to cross the river much farther downstream; this was what Ventidius hoped would occur and gave him time to get his forces ready.[4]

The Parthians faced no opposition to their river crossing and proceeded to the town of Gindarus in Cyrrhestica, confident in their belief that their Roman foes were weak or cowardly, since they did not attempt to prevent the river crossing.

When the Parthians got to the town, which sat on a small hill, they encountered Roman legions confidently formed in battle order on the slopes. The Parthians rushed to attack - whether this order came from Pacorus or was a spontaneous charge is unknown. In any case, Ventidius ordered his troops, who had the advantage of high ground, to attack the horse-archers advancing up the slope. The horse-archers were forced into close-quartered combat against the legionaries and suffered heavily for it, for they were unsuited for such combat. The Parthian cavalry's will eventually broke and panic spread, many of the horse archers being driven down the slope where they crashed into their fellows in their desperation to escape. The horse-archers eventually fled or fell. Parthian heavy cavalry, which was stationed at the bottom of the hill, was enveloped and surrounded by the legionaries.

Instead of immediately attacking with the legionaries, Ventidius made use of his slingers to rain down projectiles on the Parthian heavy cavalry, which included Pacorus himself. After the barrage was lifted the legionaries moved in and were quickly able to identify Pacorus because of his standard and expensive armor. Pacorus was eventually slain along with his bodyguards, and the remaining cavalry broke and attempted to flee from their entrapment, which not all managed to do. Overall the Roman army had achieved a complete victory.[5] Pacorus' father Orodes II, overwhelmed by the sadness about his son's death, chose now his oldest, him still remaining son Phraates IV as his successor.